1. Field of the Invention
The present invention relates to a shadow mask having a vertical pitch that simultaneously satisfies moire characteristic and purity characteristic, as distinguished from a conventional shadow mask having a vertical pitch that is equal in the shorter axis direction thereof but decreases as it goes along the longer axis direction thereof for tilt array design.
2. Background of the Related Art
Referring to FIG. 1, a conventional color cathode-ray tube (CRT) includes a panel 1 having a fluorescent screen 4 with R, G, B colors coated on the inner surface thereof and an explosion-proof glass fixed to the front portion thereof, a funnel 2 coupled to the rear panel 1, and an electron gun inserted in the neck portion of the funnel 2 for emitting electron beams 6. The color CRT further includes a deflection yoke for deflecting the electron beams, a shadow mask 3 that is mounted on the inner side of the panel at a predetermined distance therefrom and has a plurality of apertures through which the electron beams pass, and a frame 7 for fixing and supporting the shadow mask so as to maintain the predetermined distance between the shadow mask and the inner surface of the panel. The color CRT also has a spring 8, an inner shield 9 for shielding the earth-magnetic field so as to prevent the CRT from being affected by the earth-magnetic field, and a reinforcing band 11 mounted to surround the side of the panel for protecting the CRT from eternal shock. In addition, the conventional color CRT includes a magnet 10 for correcting the electron beam path so as to allow the electron beams to accurately hit predetermined phosphors, thereby preventing poor color purity.
FIG. 2 illustrates the structure of the conventional shadow mask. In FIG. 2, PVo denotes a vertical pitch between neighboring beams through apertures formed at the central portion of the shadow mask and PVc denotes a vertical pitch between adjacent beams through apertures placed at the corner portion of the shadow mask. Furthermore, PVe denotes a vertical pitch between neighboring beams through apertures formed at the end portion of the longer axis of the shadow mask, and PVt denotes a vertical pitch between adjacent beams through apertures placed at the end portion of the shorter axis of the shadow mask. PVo, PVc, PVe and PVt of the conventional shadow mask have the same value or have a difference of 1˜3% approximately.
The conventional fluorescent screen 4 is formed of R, G, B phosphors formed on the rear side thereof and arranged in a pattern of a plurality of stripes or dots and a light absorption material such as a black coating 41 formed between neighboring phosphors. An aluminum thin film 42 as a conductive layer is formed on the rear side 12 to improve the luminance of the fluorescent screen 4, to protect the fluorescent screen 4 from being ion-damaged and to prevent the fall of potential of the fluorescent screen 4.
As for the phosphors R, G and B coated on the screen, as shown in FIG. 4, the distance between G and G (or R and R) in the longer axis direction is called a horizontal pitch SP of the fluorescent screen 4 and the distance between G and G (or R and R) in the shorter axis direction is called a vertical pitch SW thereof.
The vertical pitch PV of the shadow mask 3 has a close relationship with the moire characteristic, an important characteristic of the CRT, and affects the purity characteristic and rigidity of the shadow mask 3.
The main factors in the generation of the moire phenomenon in the a CRT includes the factor affecting the moiré wavelength and the factor affecting the moire intensity. The factor affecting the moiré wavelength includes a scanning line interval of electron beams and the vertical pitch of the shadow mask. The spot size of the electron gun affects the moire intensity. The moiré wavelength is calculated through the following expression.   λ  =            1                        1                      P            ⁢                                                  ⁢            V                          -                  N                      2            ⁢                                                  ⁢            S                                ⁢                  [    mm    ]  
In this expression, S denotes the electron beam scanning line interval, PV denotes the vertical pitch of the beam through apertures and N is an integer.
The moire intensity is calculated through the following expression   M  =            k      ⁡              (                  P          ⁢                                          ⁢          V                )              ×          ⅇ              -                              A            ⁢                                                  ⁢                          D              2                        ⁢                          N              2                                            S            2                              
In this expression, k(PV) denotes a constant determined by PV, A denotes a proportional constant and D is the spot size of the electron beam.
Since moire is the most noticeable when the moire wavelength is 4 mm˜10 mm, the vertical pitch of the beam through apertures should be determined such that the moire wavelength is not in this range. In addition, the higher the moire intensity, the more pronounced the moire phenomenon. Thus, a smaller moire intensity value is preferable.
In case of the CRT, the scanning line interval depends on modes (640×480, 800×600, 1024×768, 1280×1024). Because a variation rate of the vertical pitch of the outer portion of the conventional shadow mask to that of the central portion thereof is as small as 1%˜3%, it is impossible to control the vertical pitch so as to make the moire wavelength less than 4 mm by modes.
Furthermore, occurrence of the moire phenomenon can be prevented only when the vertical pitch is controlled according to the electron beam spot size because the spot size depends on positions as shown in the above expression for calculating the moire intensity. However, this is impossible with the conventional vertical pitch variation rate.
Moreover, in the relationship among the moire characteristic, purity characteristic and shadow mask vertical pitch, the smaller the vertical pitch, the better the moire characteristic. And, the larger the vertical pitch, the better the purity characteristic.
In Korean Patent No. 97-3365, the vertical pitch of the shadow mask increases as it goes from the central portion to the end portion of the shorter axis of the shadow mask but decreases as it goes from the center to the end portion of the longer axis of the shadow mask. In Korean Patent No. 99-27074, the shadow mask vertical pitch increases as it goes from the central portion to the skirt portion of the shadow mask. However, the conventional vertical pitches were designed, giving the first consideration to the moire characteristic, so that the purity characteristic became a problem.